Abstract

Pyroxene–oxide intergrowths occur in rocks representing equilibration over a wide range of P–T–fO2 conditions; these intergrowths have three modes of occurrence: (1) nodular pyroxene–ilmenite intergrowths from kimberlite, (2) “myrmekitic” pyroxene–oxide intergrowths from cumulate rocks and mantle xenohths, and (3) fine lamellae of spinel or ilmenite in pyroxenes from cumulate rocks and mantle peridotites. Examination of three nodular clinopyroxene–ihnenite intergrowths and an ilmenite–spinel websterite from the Elliott County kimberlite, Kentucky and a cumulate norite from a small gabbroic pluton from Chester County, South Carolina has revealed intergrowths representing all three modes of occurrence. The nodular clinopyroxene–ilmenite intergrowths are interpreted to have formed during co-precipitation of Cr-poor clinopyroxene (73.0–76.8 mol.% Di) and picroilmenite (44.7–46.6 mol.% MgTiO3 and 5.4–5.8 mol.% Fe2O3) at 1295°–1335°C from the “proto-kimberlitic” hquid which produced the Elliott County kimberlite megacryst suite. The Elliott County websterite xenolith contains fine (1–5 μm) lamellae of magnetite (12.6 mol.% (Fe,Mg)Al2O4 and 8.0 mol.% (Fe,Mg)2TiO4) and pleonaste (6.8 mol.% (Fe,Mg)Fe2O4 and 0.1 mol.% (Fe,Mg)2TiO4) within aluminous clinopyroxene (8.5 mol.% CaTs and 7.3 mol.% Jd) and orthopyroxene (7.8 mol.% MgTs); these intergrowths are interpreted to have formed during “exsolution” of spinel from high-temperature (1105°–1140°C), non-stoichiometric pyroxenes. Coexisting oxides suggest equilibration at 695°C and fO2 = 10−13.8. The Chester cumulate norite contains both ilmenite lamellae (11.6 mol.% MgTi03 and 5.7 mol.% Fe2O3) in orthopyroxene (4.4 mol.% MgTs) and clinopyroxene (5.6 mol.% CaTs and 2.7 mol.% Jd) and myrmekitic intergrowths of orthopyroxene and magnetite (0.8 mol.% (Fe,Mg)Al2O4 and 0.5 mol.% (Fe,Mg)2TiO4) and minor ilmenite (14.3 mol.% MgTiO3 and 3.8 mol.% Fe2O3). The lamellar intergrowths were produced during “exsolution” from high-temperature (1200°C), non-stoichiometric pyroxenes. Myrmekitic intergrowths formed during co-precipitation of orthopyroxene, magnetite, and ilmenite. Coexisting Fe–Ti oxides suggest equilibration temperatures of 480°–515°C at fO2 = 10−21.0−10−23.7.

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